Communication terminal apparatus, network system and handover method

ABSTRACT

A communication terminal apparatus wherein even if there occurs a dropout or overtaking of call control messages, the current service can be continued without increasing the load of an apparatus such as a server in the network. In this apparatus, a VoIP call control part ( 103 ) performs call controls including both a call establishment of a VoIP telephone conversation using a wireless LAN system and a call disconnection thereof. A line switching call control part ( 104 ) performs call controls including both a call establishment of a line switching telephone conversation using a mobile communication (cellular) system and a call disconnection thereof. A handover part ( 105 ) determines the reception quality of the wireless LAN system and that of the mobile communication (cellular) system and controls, based on the determination results, the handover between the wireless LAN system and the mobile communication (cellular) system.

TECHNICAL FIELD

The present invention relates to a communication terminal apparatus, anetwork system and a handover method. More particularly, the presentinvention relates to a communication terminal apparatus, a networksystem and a handover method that allows connection with networksadopting different communication schemes and handover between suchnetworks.

BACKGROUND ART

Presently, system development has proceeded in systems where radiocommunication terminals freely perform handover between systems of amobile communication (cellular) system which allows a lower datatransmission rate yet covers a wider area and a wireless LAN systemwhich allows a higher data transmission rate yet covers a smaller area.Particularly, in 3GPP (3rd Generation Partnership Project) planningcommunication standards for mobile communications, studies are underwayto standardize a handover scheme capable of continuing voicecommunication between circuit-switched voice communication and VoIPvoice communication (see, 3GPP draft TR (Technical Report) 23.806, TS(Technical Specification) 23.206). Further, the technique disclosed inPatent Document 1 is known as a handover scheme between circuit-switchedvoice communication and VoIP voice communication.

To perform handover between a circuit-switched network supportingcircuit-switched voice communication and an IP network supporting VoIPvoice communication while enjoying telephone service, as shown in FIG.12 of Patent Document 1, the steps of establishing a call for the targetdomain and then disconnecting the call from the source domain are used.In Patent Document 1, terminals calling each other perform call control,and, as studied in 3GPP TR 23.806, by relaying a call by one server or aplurality of servers in a network and by converting call controlmessages and converting medias between circuit-switched voice media andVoIP voice media, it is possible that handover is performed regardlessof the call connection schemes communicating parties support.

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2004-297591

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in the conventional system studied in 3GPP TR 23.806, thenetwork configuration becomes complicated, and therefore there may becases where the steps of establishing a call with the target domain andthen disconnecting the call with the source domain cannot be executedsequentially, as shown in FIG. 12 of Patent Document 1. In that case,due to loss of call control messages and the difference of thetransmission delay between a circuit-switched network and an IP networkused in VoIP, voice communication is suspended and the time voice isinterrupted increases.

For example, in a situation where a call control message for the targetdomain network is lost and a call for a target domain network is notestablished, if a call from the source domain is disconnected, there isa problem that voice communication is suspended. Further, due totransmission delay of the network, if a call from the source domain isdisconnected before a call control message for the target domain networkis established, there is a problem that the time required for handoverincreases and the time voice is interrupted increases.

FIGS. 1 to 3 are sequence diagrams showing handover schemes studied in3GPP supporting voice communication continuing between circuit-switchedvoice communication and VoIP voice communication.

FIG. 1 shows a sequence diagram showing a case where handover isexecuted normally, and VoIP voice communication switches tocircuit-switched voice communication. In FIG. 1, a radio communicationterminal transmits a setup message, which requests a handover (step ST11, step ST 12, step ST 13 and step ST 14), and a handover managementserver having received the request for a handover transmits a messagefor establishing a call with the target domain (step ST 15, step ST 16,step ST 17 and step ST 18). Then, the radio communication terminalhaving received a connect message, which is for establishing the callwith the target domain, transmits a connect Ack message, which is aresponse to the connect message (step ST 19). Then, the handovermanagement server receives the response to the message for establishinga call with the target domain (step ST 20 and step ST 21) and transmitsa bye message, which requests disconnection of the call with the sourcedomain (step ST 22 and step ST 23). Then, the radio communicationterminal having received the bye message transmits a 200 OK message,which is a response to the bye message (step ST 24 and step ST 25).Here, steps ST 11 to ST 19 form a sequence for establishing a call forcircuit-switched, which is the target domain, and steps ST 22 to ST 25form a sequence for disconnecting the call from a VoIP, which is thesource domain. FIG. 1 only describes apparatuses and messages related tothe present invention.

FIG. 2 is a sequence diagram showing a case where a connect message,which is circuit-switched call control, is lost over the radio. Afterreceiving the 200 OK message transmitted from the SIP (Session InitialProtocol) server (step ST 16), the call control switch server transmitsan ANM (Answer Message, step ST 17) to a circuit-switched server in amobile communication circuit-switched network and at the same timetransmits an Ack to the SIP server (step ST 20 and ST 21), so that, thehandover management server transmits a bye message to SIP serverregardless of whether the transmission of a connect message issuccessful or failed (step ST 22 and step ST 23). By transmitting andreceiving the bye message and the 200 OK message, which is a response tothe bye message, VoIP voice communication is disconnected. If the radiocommunication terminal cannot receive the connect message due to badchannel condition or if the connect message is tried to be retransmittedbut failed between the circuit-switched server and the radiocommunication terminal, the VoIP call is disconnected in a state where acircuit-switched call is not established, and so communication is notpossible.

FIG. 3 shows a sequence diagram showing a case where a bye message,which is call control, is received and later a connect message, which iscall control, is received (i.e. outrun). By the operation ofretransmitting the connect message due to transmission delay in thecircuit-switched network or poor radio environment, it is possible thata connect message arrives at the radio communication terminal (step ST50 and step ST 51) later than the bye message via the IP network (stepST 22 and step ST 23). In this case, circuit-switched voicecommunications tarts after VoIP voice communication is disconnected, andso the time voice is interrupted increases.

Neither 3GPP TR 23.806 nor Patent Document 1 makes clear operations in acase where call control messages are lost or where call control messagesare outrun. Further, there are cases where call control is conducted indifferent networks, and so handshaking communication cannot be carriedout, and, when messages are lost, the above problem occurs. In thiscase, it is possible to cope with apparatuses such as servers in thenetworks, but modification of existing systems and protocols mayincrease, thereby increasing the cost.

The present invention is made in view of the above-described problems,and it is therefore an object of the present invention to provide acommunication terminal apparatus, a network system and a handover methodthat make it possible to continue services without increasing loads ofapparatuses such as servers in networks, even when call control messagesare lost and call control messages are outrun.

Means for Solving the Problem

The communication terminal apparatus of the present invention adopts aconfiguration including: a call control section that performs callcontrol including call establishment and call disconnection for eachnetwork adopting a different communication scheme; a received qualitymeasurement section that measures received quality of said each networkadopting a different communication scheme; and a handover controlsection that, in a case where a handover is performed between thenetworks adopting different network schemes based on the measuredreceived quality, when a call with a target domain is not established inthe call control section, makes the call control section not disconnecta call with a source domain and requests a repetition of the handover.

The network system of the present invention adopts a configurationincluding: a communication terminal apparatus that performs a handoverbetween networks adopting different communication schemes based onreceived quality in the networks adopting different communicationschemes, and, when a call with a target domain is not established uponthe handover, makes the call with a source domain not disconnect andrequests a repetition of the handover; and a handover management serverthat requests disconnection of the call with the source domain byreceiving a request for a handover from the communication terminalapparatus and requests establishment of a call with the target domain.

The handover method of the present invention includes steps of:performing call control including call establishment and calldisconnection for each network adopting a different communicationscheme; received quality of said each network adopting a differentcommunication scheme; and in a case where a handover is performedbetween the networks adopting different network schemes based on themeasured received quality, requesting a repetition of the handoverwithout disconnecting a call with a source domain when a call is notestablished with a target domain.

Advantageous Effect of the Invention

According to the present invention, it is possible to continue serviceswithout increasing loads of apparatuses such as servers in networks,even when call control messages are lost and call control messages areoutrun.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a sequence diagram showing a conventional handoverscheme;

FIG. 2 illustrates a sequence diagram showing a conventional handoverscheme;

FIG. 3 illustrates a sequence diagram showing a conventional handoverscheme;

FIG. 4 is a block diagram showing the configuration of the communicationterminal apparatus, according to Embodiment 1 of the present invention;

FIG. 5 is a block diagram showing the configuration of a handovermanagement server, according to Embodiment 1 of the present invention;

FIG. 6 is a block diagram showing the configuration of a handoversection, according to Embodiment 1 of the present invention;

FIG. 7 shows the network configuration, according to Embodiment 1 of thepresent invention;

FIG. 8 illustrates a sequence diagram showing the handover method,according to Embodiment 1 of the present invention;

FIG. 9 illustrates a sequence diagram showing the handover method,according to Embodiment 2 of the present invention;

FIG. 10 illustrates a sequence diagram showing the handover method,according to Embodiment 3 of the present invention;

FIG. 11 illustrates a sequence diagram showing the handover method,according to Embodiment 3 of the present invention; and

FIG. 12 illustrates a sequence diagram showing the handover method,according to Embodiment 4 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, embodiments of the present invention will be described in detailwith reference to the accompanying drawings.

Embodiment 1

FIG. 4 is a block diagram showing the configuration of communicationterminal apparatus 100 according to Embodiment 1 of the presentinvention. In FIG. 4, wireless LAN IF 101, circuit-switched IF 102, VoIPcall control section 103, circuit-switched call control section 104,handover section 105 and input/output section 106 are connected by bus107.

Wireless LAN IF 101 receives a VoIP voice communication signal by awireless LAN system, and outputs the received signal to VoIP callcontrol section 103, handover section 105 and input/output section 106,via bus 107. Further, wireless LAN IF 101 transmits signals of VoIPvoice communication by a wireless LAN system, inputted from VoIP callcontrol section 103, handover section 105 and input/output section 106via bus 107. Further, wireless LAN IF 101 transmits a message for callestablishment inputted from VoIP call control section 103, and outputs areceived message for call disconnection to VoIP call control section103. Further, wireless LAN IF 101 transmits a message for requesting ahandover inputted from handover section 105.

Circuit-switched IF 102 receives a circuit-switched voice communicationsignal by a mobile communication (cellular) system, and outputs thereceived signal to circuit-switched call control section 104, handoversection 105 and input/output section 106, via bus 107. Further,circuit-switched IF 102 transmits signals of circuit-switched voicecommunication by a mobile communication (cellular) system inputted fromcircuit-switched call control section 104, handover section 105 andinput/output section 106 via bus 107. Further, circuit-switched IF 102transmits a message for call establishment inputted fromcircuit-switched call control section 104, and outputs a receivedmessage for call disconnection to circuit-switched call control section104. Further, circuit-switched IF 102 transmits a message for requestinga handover inputted from handover section 105. In this case, the VoIPvoice communication by the wireless LAN system and the circuit-switchedvoice communication by the mobile communication (cellular) system referto networks adopting different communication schemes.

VoIP call control section 103, which is a call control means, outputsmessages related to call establishment and call disconnection towireless LAN IF 101 via bus 107, and receives as inputs of messagesrelated to call establishment and call disconnection from wireless LANIF 101 via bus 107, thereby performing call control of callestablishment and call disconnection of VoIP voice communication by awireless LAN system. Further, if VoIP call control section 103 receivesa command from handover section 105 to disable a call disconnectionrequest, VoIP call control section 103 disables a message for requestinga call disconnection inputted from wireless LAN IF 101, to prevent acall from disconnection.

Circuit-switched call control section 104, which is a call control meansoutputs the message related to call establishment and call disconnectionto circuit-switched IF 102 via bus 107, and receives as input of themessages related to call establishment and call disconnection fromcircuit-switched IF 102 via bus 107, thereby performing call control ofcall establishment and call disconnection of circuit-switched voicecommunication by a mobile communication (cellular) system. Further, ifcircuit-switched call control section 104 receives a command fromhandover section 105 to disable a call disconnection request,circuit-switched call control section 104 disables a message forrequesting a call disconnection inputted from circuit-switched IF 102,to prevent a call from disconnection.

Handover section 105 monitors the state of call control conducted byVoIP control section 103, the state of a call established by the callcontrol and the state of a call disconnected by the call control, andmonitors the state of call control conducted by circuit-switched callcontrol section 104, the state of a call established by the call controland the state of a call disconnected by the call control. Further,handover section 105 measures the received quality of the wireless LANsystem from the signal inputted from wireless LAN IF 101 and thereceived quality of the mobile communication (cellular) system from thesignal inputted from circuit-switched IF 102, and controls the handoverbetween the wireless LAN system and the mobile communication (cellular)system based on the measurement results. Further, after a handoverrequest accompanying the handover control, if VoIP call control section103 or circuit-switched call control section 104 receives the messagefor requesting disconnection of the call with the source domain and if acall with the target domain is not established, handover section 105commands VoIP call control section 103 or circuit-switched call controlsection 104 to disable the request to disconnect the call with thesource domain. The configuration of handover section 105 will beexplained later in detail.

By establishing a call selected in handover section 105, input/outputsection 106 provides a user with various services received at wirelessLAN IF 101 or circuit-switched IF 102 and inputted via bus 107, throughinput/output apparatuses such as a microphone, a receiver, speakers or adisplay.

Next, the configuration of handover management server 200 will beexplained using FIG. 5. FIG. 5 shows a block diagram showing theconfiguration of handover management server 200.

Communication IF 201, source domain call control section 202, targetdomain call control section 203 and handover central management section204 are connected by bus 205.

Communication IF 201 receives a VoIP voice communication signal by awireless LAN system and a circuit-switched voice communication signal bya mobile communication (cellular) system, and outputs the receivedsignals to source domain call control section 202, target domain callcontrol section 203 and handover central management section 204, via bus205. Further, communication IF 201 transmits the signals inputted fromsource domain call control section 202, target domain call controlsection 203 and handover central management section 204, via bus 205.Further, communication IF 201 outputs the received message forrequesting a handover to handover central management section 204, andtransmits a message for requesting disconnection of the call with thesource domain inputted from source domain call control section 202 and amessage for requesting establishment of a call with the target domain.

Source domain call control section 202 controls calls in the sourcedomain. To be more specific, according to the command of handovercentral management section 204, source domain call control section 202outputs the message for requesting disconnection of the call with thesource domain, to communication IF 201.

Target domain call control section 203 controls calls in the targetdomain. To be more specific, according to the command of handovercentral management section 204, target domain call control section 203outputs a message for requesting establishment of a call with the targetdomain, to communication IF 201.

Handover central management section 204 monitors the state of the callcontrol conducted by source domain call control section 202 and thestate of the call control conducted by target domain call controlsection 203, and controls a handover. To be more specific, if a messagefor requesting a handover is inputted from communication IF 201,handover central management section 204 commands source domain callcontrol section 202 to output a message for requesting disconnection ofthe call with the source domain and commands target domain call controlsection 203 to output a message for requesting establishment of a callwith the target domain.

Next, handover section 105 of communication terminal apparatus 100 willbe explained in detail using FIG. 6. FIG. 6 is a block diagram showingthe configuration of handover section 105.

Call monitoring section 301 monitors call disconnection or callestablishment in VoIP call control section 103 and circuit-switched callcontrol section 104. Further, call monitoring section 301 monitorswhether or not a message for requesting disconnection of a call isinputted to VoIP call control section 103 and circuit-switched callcontrol section 104. Then, call monitoring section 301 outputs themonitor results to handover control section 303.

Received quality measurement section 302 measures the received qualityof a VoIP voice communication signal by the wireless LAN system and thereceived quality of a circuit-switched voice communication signal by themobile communication (cellular) system. Then, received qualitymeasurement section 302 outputs the measurement results to handovercontrol section 303.

Based on the measurement results of the received quality inputted fromreceived quality measurement section 302, handover control section 303determines whether or not to perform handover between the wireless LANsystem and the mobile communication (cellular) system. Then, upondetermining performing the handover, handover control section 303creates a message for requesting a handover, and outputs the createdmessage to wireless LAN IF 101 or to circuit-switched IF 102. Further,upon determining performing the handover, handover control section 303refers to the monitor results inputted from call monitoring section 301,and, if VoIP call control section 103 or circuit-switched call controlsection 104 receives a message for requesting disconnection of the callwith the source domain and a call with the target domain is notestablished, handover control section 303 commands VoIP call controlsection 103 or circuit-switched call control section 104 to disable thereceived message for requesting call disconnection.

FIG. 7 shows the configuration of network 400 according to Embodiment 1.

Network 400 has communication terminal apparatus 401, IP network 402,circuit-switched network 403, integration network 404, network 405 andcommunicating party 406. Further, IP network 402 includes wireless LANaccess point 407 and SIP server 408. Further, circuit-switched network403 includes base station 409 and circuit-switched server 410. Further,integration network 404 includes handover management server 411, callcontrol conversion server 412 and media gateway 413.

Communication terminal apparatus 401, which has the same configurationas communication terminal apparatus 100 shown in FIG. 4 and FIG. 6, iscapable of circuit-switched voice communication and VoIP voicecommunication.

Communicating party 406 is a communication apparatus conducting voicecommunication with communication terminal apparatus 401 through network405. The communication method for which communicating party 406 conductsvoice communication is not particularly limited.

Wireless LAN access point 407 is provided because communication terminalapparatus 401 connects to an IP network and pursuant to IEEE 802.11a, band g and so on.

SIP server 408 performs VoIP call control (here, SIP).

Base station 409 is provided to allow communication terminal apparatus401 to connect to a circuit-switched network.

Circuit-switched server 410 performs circuit-switched call control in acircuit-switched network.

Handover management server 411, which has the same configuration ashandover management server 200 shown in FIG. 5, manages handover callcontrol. Further, handover management server 411 collectively manages acircuit-switched call control message and a VoIP call control message.In Embodiment 1, handover management server 411 converts acircuit-switched call control message into a VoIP call control (SIP)message, and processes the converted message.

Call control conversion server 412 converts between a circuit-switchedcall control message and a VoIP call control message.

Media gateway 413 relays a call (voice media) with communicating party406 and converts the circuit-switched or VoIP voice media whichcommunication terminal apparatus 401 transmits, into voice media whichcommunicating party 406 can receive. Further, media gateway 413 convertsthe voice media communicating party 406 transmits, into circuit-switchedvoice media or VoIP voice media.

Next, handover method in network 400 will be explained using FIG. 8.FIG. 8 is a sequence diagram showing the handover method.

In FIG. 8, communication terminal apparatus 401 establishes a VoIP voicecommunication call by an SIP via an IP network and conducts VoIP voicecommunication, and has finished the registration for continuingreceiving VoIP voice communication services. Further, the voice media isrelayed and format-converted by the media gateway, and, if a handoveroccurs, call control is conducted between communication terminalapparatus 401 to perform handover and an apparatus in the network.

Due to the reasons of bad channel conditions between communicationterminal apparatus 401 and wireless LAN access point 407 and goodchannel conditions between communication terminal 401 and base station409, and so on, handover section 105 of communication terminal apparatus401 determines to perform handover from VoIP voice communication via IPnetwork 402 to circuit-switched voice communication via circuit-switchednetwork 403. To be more specific, received quality measurement section302 of handover section 105 measures the received quality of a VoIPvoice communication signal by the wireless LAN system and the receivedquality of a circuit-switched voice communication signal by the mobilecommunication (cellular) system. Then, from the measurement results ofreceived quality, handover control section 303 of handover section 105can know bad channel conditions between communication terminal apparatus401 and wireless LAN access point 407, and good channel conditionsbetween communication terminal 401 and base station 409. If the receivedquality of the VoIP voice communication signal by the wireless LANsystem becomes poorer than a threshold value and the received quality ofthe circuit-switched voice communication signal by the mobilecommunication (cellular) system is equal to and more than a thresholdvalue, handover control section 303 of handover section 105 determinesto perform handover from VoIP voice communication via IP network 402 toa circuit-switched voice communication via circuit-switched network 403.

Next, handover control section 303 of handover section 105 transmits asetup message, which starts circuit-switched call processing andrequests a handover, via circuit-switched IF 102 (step ST 501).

Next, circuit-switched server 410 receives the setup message via basestation 409, acknowledges a handover command from communication terminalapparatus 401, and transmits an IAM message, which is a handovercommand, to handover management server 411 (step ST 502). At this time,a setup message, which starts circuit-switched call processing, isconverted into another call control message and then transferred.

Next, call control conversion server 412 converts an IAM message, whichis a handover command, into an INVITE message, and transmits the INVITEmessage to handover management server 411 (step ST 503).

Next, handover management server 411 receives an INVITE, which is amessage for a handover command. Handover central management section 204of handover management server 411 commands target domain call controlsection 203 to establish a call for a circuit-switched network, which isthe target domain. Then, target domain call control section 203transmits a 200 OK, which is a message for connection notification forestablishing a call for the circuit-switched network, which is thetarget domain network, via communication IF 201 (step ST 504).

Next, call control conversion server 412 converts a 200 OK, which is aconnection notification message for establishing a circuit-switchedcall, into an ANM, and transmits the ANM to circuit-switched server 410(step ST 505).

Next, circuit-switched server 410 receives the ANM, and transmits aconnect message, which is a connection notification for establishingcall for a circuit-switched network, which is the target domain, viabase station 409. However, communication terminal apparatus 401 cannotreceive the connect message because of bad channel conditions betweenbase station 409 and terminal communication apparatus 401 (step ST 506).

Further, if communication terminal apparatus 401 cannot receive aconnect message, which is a circuit-switched connection notification,call control conversion server 412 transmits an Ack message, which is anacknowledgement in response to a 200 OK message, which is a connectionnotification (step ST 507).

Next, handover management server 411 receives the Ack, which is anacknowledgment message. Handover central management section 204 ofhandover management server 411 commands source domain call controlsection 202 to disconnect the call from VoIP voice communication, whichis the source domain. Then, to disconnect the already established callfrom VoIP voice communication, which is the source domain, source domaincall control section 202 of handover management server 411 transmits abye message, which is a disconnection request for disconnecting VoIPvoice communication via communication IF 201 (step ST 508).

Next, SIP server 408 having received the bye message, which is adisconnection request, transmits the bye message, which is adisconnection request, via wireless LAN access point 407 (step ST 509).

Next, communication via wireless LAN is correctly carried out, and callmonitoring section 301 of handover section 105 acknowledges that VoIPcall control section 103 has received the bye message.

Next, call monitoring section 301 learns that a circuit-switched voicecommunication call is not established because circuit-switched callcontrol section 104 has not received the connect message.

Next, handover control section 303 commands VoIP call control section103 to disable the received bye message, and creates a message forrequesting a handover again. At this time, handover control section 303can use one of the method of creating the same message as the messagecreated for the first time requesting a handover, the method of creatinga message including an identifier indicating that the handover requestis not for the first time, and the method of creating a specific messagefor requesting a handover again instead of a setup message.

Next, circuit-switched IF 102 of communication terminal apparatus 401transmits a message for requesting a handover again created by handovercontrol section 303 (step ST 510).

Next, handover central management section 204 of handover managementserver 411 learns source domain call control section 202 has transmitteda bye message. Then, upon receiving a command message for a handover ina state where source domain call control section 202 has established acall with the source domain, handover central management section 204commands target domain call control section 203 to transmit a 200 OKmessage, which is a connection notification, and, target domain callcontrol section 203 transmits the 200 OK message, which is a connectionnotification, via communication IF 201 (step ST 511).

Next, call control conversion server 412 converts a 200 OK, which is aconnection notification message for establishing a circuit-switchedcall, into an ANM, and transmits the ANM to circuit-switched server 410(step ST 512).

Next, circuit-switched server 410 receives the ANM, and transmits aconnect message, which is a connection notification for establishing acall for a circuit-switched network, which is the target domain, viabase station 409 (step ST 513).

Next, call control conversion server 412 in the network transmits an Ackmessage, which is an acknowledgement in response to a 200 OK message,which is a connection notification (step ST 514).

Next, communication terminal apparatus 401 having received the connectmessage transmits the connect ACK message, which is an acknowledgementin response to the connect message (step ST 515).

Handover central management section 204 of handover management server411 commands source domain call control section 202 to transmit a byemessage, which is a disconnection request, and, source domain callcontrol section 202 transmits a bye message, which is a disconnectionrequest (step ST 516).

Next, SIP server 408 having received a bye message, which is adisconnection request, transmits a bye message, which is a disconnectionrequest, via wireless LAN access point 407 (step ST 517).

Next, the communication via wireless LAN is correctly carried out, andcall monitoring section 301 of handover section 105 learns that VoIPcall control section 103 has received the bye message.

Next, call monitoring section 301 acknowledges that a circuit-switchedvoice communication call is not established because circuit-switchedcall control section 104 has received the connect message.

Next, communication terminal 401 transmits a 200 OK message, which is anacknowledgement in response to the bye message, which is a disconnectionrequest (step ST 518).

Next, SIP server 408 receives a 200 OK message, and transmits the 200 OKmessage to handover management server 411 (step ST 519).

By receiving the 200 OK message, handover management server 411 is ableto learn communication terminal apparatus 401 has succeeded in thehandover.

Although coordination with the networks is necessary, the switching ofmedia by handover section 105 of communication terminal apparatus 401may be carried out after a connect message is received in ST 513, thatis, after a call with the target domain is established, before a 200 OKmessage is transmitted in ST 518, that is, before the call with thesource domain is disconnected.

In this way, according to Embodiment 1, in a case where a request todisconnect a call with the source domain is received and yet a call withthe target domain is not established, a request to disconnect the callwith the source domain is disabled and a handover is requested again, sothat, even when call control messages are lost or when call controlmessages are outrun, it is possible to prevent a circuit-switchednetwork call and a IP network call from being disconnected at the sametime and continue providing services without increasing loads ofapparatuses such as servers in the networks. Further, according toEmbodiment 1, the radio communication terminal is allowed to detect alost or outrun message earlier than the servers in the network, so thatit is possible to prevent state of servers in networks and a state ofthe radio communication terminal from being mismatched. Further,according to Embodiment 1, if, as a message for requesting a handoveragain, a message including an identifier indicating that the handoverrequest is not for the first time, a handover can be executed again,and, a receiving side receiving a message for requesting a handoveragain can easily determines whether the received message is a handoverrequest for the first time or a handover request again. Further,according to Embodiment 1, it is possible to perform reliable handoverwithout changing predefined protocol requirements such that, in step ST515, when circuit-switched server 410 receives the connect Ack message,circuit-switched server 410 transmits a response message in response toan ANM, to call control conversion server 412.

With Embodiment 1, although a handover for a second time is requested bytransmitting a circuit-switched message via the circuit-switchednetwork, which is the target domain, Embodiment 1 is not limited tothis, and a handover for a second time may be requested by transmittinga VoIP message via an IP network, which is the source domain.

Embodiment 2

FIG. 9 a sequence diagram showing the handover method according toEmbodiment 2 of the present invention. In FIG. 9, the same numbers areassigned to the same steps as in FIG. 8 and the description thereof willbe omitted. Further, in Embodiment 2, the configuration of thecommunication terminal apparatus is the same as in FIGS. 4 and 6, theconfiguration of the handover management server is the same as in FIG.5, and the network configuration is the same as in FIG. 7, and thedescription thereof will be omitted.

Features of Embodiment 2 include using both a wireless LAN system and amobile communication (cellular) system in handover control section 303upon requesting a handover again.

SIP server 408 having received a bye message, which is a disconnectionrequest, transmits the bye message, which is a disconnection request viawireless LAN access point 407 (step ST 509).

Next, the communication via wireless LAN is correctly carried out, andcall monitoring section 301 of handover section 105 learns that VoIPcall control section 103 has received the bye message.

Next, call monitoring section 301 acknowledges that a circuit-switchedvoice communication call is not established because circuit-switchedcall control section 104 has not received the connect message.

Next, handover control section 303 creates a message for requesting ahandover from VoIP voice communication to circuit-switched voicecommunication. At this time, handover control section 303 creates both aVoIP call control message via IP network 402 and a circuit-switched callcontrol message via circuit-switched network 403.

Next, as a message for requesting a handover again, circuit-switched IF102 of communication terminal apparatus 401 transmits a circuit-switchedcall control message via circuit-switched network 403 (step ST 601)wireless LAN IF 101 of communication terminal apparatus 401 transmits aVoIP call control message via IP network 402 (step ST 602).

Next, handover central management section 204 of handover managementserver 411 acknowledges source domain call control section 202 hastransmitted the bye message. Then, upon receiving a command message fora handover in a state where source domain call control section 202 hasestablished a call with the source domain, handover central managementsection 204 commands target domain call control section 203 to transmita 200 OK message, which is a connection notification, and, target domaincall control section 203 transmits the 200 OK message, which is aconnection notification (step ST 511).

In this way, according to Embodiment 2, in addition to theabove-described advantage of Embodiment 1, upon requesting a handoveragain, both a message for requesting a second circuit-switched handovervia circuit-switched network 403, which is a target domain, and amessage for requesting a handover again used for VoIP via IP network402, which is a source domain, are transmitted, so that it is possibleto reliably report that the target domain network has not establishedand it is likely that a handover for a second time is successful.

Embodiment 3

FIGS. 10 and 11 are sequence diagrams showing the handover methodaccording to Embodiment 3 of the present invention. In FIGS. 10 and 11,the same numbers are assigned to the same steps as in FIG. 8 and thedescription thereof will be omitted. Further, in Embodiment 3, theconfiguration of the communication terminal apparatus is the same as inFIGS. 4 and 6, the configuration of the handover management server isthe same as in FIG. 5, and the network configuration is the same as inFIG. 7, and the description thereof will be omitted.

Features of Embodiment 3 include requesting a handover again when acommunication terminal apparatus fails to receive a message forrequesting establishment of a call with the target domain within apredetermined period.

First, a case where communication terminal apparatus 401 receives aconnect message with delay, will be explained using FIG. 10.

SIP server 408 having received a bye message, which is a disconnectionrequest, transmits the bye message, which is a disconnection request,via wireless LAN access point 407 (step ST 509).

Next, communication terminal apparatus 401 receives the bye message, andfor a given time period α (call establishment waiting period) fromreceiving the bye message in communication terminal apparatus 100,handover control section 303 of handover section 105 waits a monitorresult in an event that circuit-switched call control section 104 hasreceived a call connection notification message for the target domain,from call monitoring section 301.

Next, communication terminal apparatus 401 receives a connect message,which is a call connection notification for the target domain and whichis a circuit-switched call control message, in the given time period, α(step ST 701).

Next, by receiving a monitor result from call monitoring section 301 inan event that circuit-switched call control section 104 has received aconnect message, handover control section 303 of handover section 105knows that call control section 104 has received a connect message.

Next, communication terminal apparatus 401 transmits an connect Ackmessage, which is an acknowledgement in response to the connect message(step ST 702).

Next, wireless LAN IF 101 of communication terminal apparatus 401transmits a 200 OK message, which is a response to the bye message,which is a VoIP call control message (step ST 703).

Next, handover management server 411 receives the 200 OK message, and,the bye message has been transmitted in step ST 508, and then handovercentral management section 204 of handover management server 411 knowsto have received the 200 OK message in a predetermined period β (β>α).Then, source domain call control section 202 of handover managementserver 411 commands source domain call control section 202 to disconnectthe VoIP voice communication call, and source domain call controlsection 202 disconnects the VoIP voice communication call.

Although coordination with the networks is necessary, the switching ofmedia by handover section 105 of a communication terminal apparatus,that is, handover, may be carried out after a connect message isreceived in step ST 701 and a call is established with the targetdomain, before a 200 OK message transmission in step ST 703 makes thecall with the source domain to be disconnected.

Next, a case where communication terminal apparatus 401 has not receiveda connect message will be explained using FIG. 11.

SIP server 408 having received a bye message, which is a disconnectionrequest, transmits the bye message, which is a disconnection request,via wireless LAN access point 407 (step ST 509).

Next, communication terminal apparatus 401 receives the bye message, andfor the given time period α from receiving the bye message incommunication terminal apparatus 100, handover control section 303 ofhandover section 105 waits a monitor result in an event thatcircuit-switched call control section 104 has received a call connectionnotification message for the target domain, inputted from callmonitoring section 301.

Then, handover control section 303 of handover section 105 knows thatcircuit-switched call control section 104 has not received a connectmessage, which is a call connection notification for the target domainin the given time period α, disables the bye message, which is adisconnection request that VoIP call control section 103 has received instep ST 509, and transmits a handover command message again (step ST801).

In this way, according to Embodiment 3, in addition to theabove-described advantage of Embodiment 1, a handover is performed if aconnection notification message is received until a given time period αpasses, so that it is possible to switch media even when the callconnection notification message for a target domain is received laterthan a call disconnection request message from a source domain. Further,according to Embodiment 3, time period α is set shorter than time periodβ, so that, in a case where the communication terminal apparatus hasalready established a call with the target domain, it is possible toprevent a handover management server from incorrectly giving upestablishing a call for the target domain.

In Embodiment 3, handover control section 303 of handover section 105can prevent communication from being suspended by adjusting a timeperiod α and shorten the time required for a handover. Further, inEmbodiment 3, handover central management section 204 of handovermanagement server 411 can prevent communication from being suspended byadjusting time period β. For example, in a case where handover section105 of the communication terminal apparatus knows not to have received aconnect message in the time period α, when handover section 105determines that VoIP voice communication has already been disconnectedby detecting that voice media is not received, handover section 105shorten the time period α. On the other hand, in a case of receiving aconnect message immediately after time period α passes, handover controlsection 303 makes the time period α longer.

Further, with Embodiment 3, although, a handover for a second time isrequested by transmitting a circuit-switched message via thecircuit-switched network of the target domain, Embodiment 3 is notlimited to this, and, a handover for a second time may be requested bytransmitting a message used for VoIP via the IP network of the sourcedomain.

Embodiment 4

FIG. 12 a sequence diagram showing the handover method according toEmbodiment 4 of the present invention. In FIG. 12, the same numbers areassigned to the same steps as in FIG. 8 and the description thereof willbe omitted. Further, in Embodiment 4, the configuration of thecommunication terminal apparatus is the same as in FIGS. 4 and 6, theconfiguration of the handover management server is the same as in FIG.5, and the network configuration is the same as in FIG. 7, and thedescription thereof will be omitted.

Features of Embodiment 4 include quitting a handover when thecommunication terminal apparatus receives a call disconnection requestwith the source domain and does not establish a call with the targetdomain.

SIP server 408 having received a bye message, which is a disconnectionrequest, transmits the bye message, which is a disconnection request,via wireless LAN access point 407 (step ST 509).

Next, the communication via wireless LAN is correctly carried out, andcall monitoring section 301 of handover section 105 learns that VoIPcall control section 103 has received the bye message.

Next, call monitoring section 301 acknowledges that a circuit-switchedvoice communication call is not established because circuit-switchedcall control section 104 has not received the connect message.

Next, handover control section 303 commands VoIP call control section103 to disable the received bye message, and creates a message forquitting the handover.

Next, circuit-switched IF 102 of communication terminal apparatus 401transmits the message for quitting the handover created by handovercontrol section 303 (step ST 901).

Next, handover management server 411 receives the message for quittingthe handover, handover central management section 204 commands sourcedomain call control section 202 not to disconnect the VoIP call fromsource domain and commands target domain call control section 203 togive up establishing a call with the target domain. This enablescommunication terminal apparatus 401 to continue using the call from thesource domain.

In this way, according to Embodiment 4, in a case where a request todisconnect a call with the source domain is received and yet a call withthe target domain is not established, a request to disconnect the callwith the source domain is disabled and a handover is quit, so that it ispossible to prevent both a circuit-switched network call and an IPnetwork call from being disconnected at the same time and continueproviding services without increasing loads of apparatuses such asservers in the networks. Further, according to Embodiment 4, the radiocommunication terminal is allowed to detect a lost or outrun messageearlier than the servers in the network, so that it is possible toprevent state of servers in networks and a state of the radiocommunication terminal from being mismatched.

With Embodiment 4, although, a handover is quit by transmitting acircuit-switched message via the circuit-switched network of the targetdomain, Embodiment 4 is not limited to this, and, a handover may be quitby transmitting a VoIP message via the IP network of the source domain.

Although cases have been explained above with Embodiments 1 to 4 wherehandover is performed from VoIP communication by SIP to circuit-switchedvoice communication, the present invention is not limited to this andmay be applied to services where handover is performed fromcircuit-switched voice communication to VoIP communication by SIP, andwhere handover is performed between different call control networks toenable handover even when, for example, packet loss occurs. Further, inthe above-described Embodiments 1 to 4, when the communication terminalapparatus does not receive a connect message in response to a requestfor a handover again and nevertheless receives a bye message, thecommunication terminal apparatus may request a handover once again, or,if the communication terminal apparatus has repeated requesting ahandover over and over again and nevertheless does not receive a connectmessage, the communication terminal apparatus may quit a handover.

INDUSTRIAL APPLICABILITY

The communication terminal apparatus, network system and handover methodaccording to the present invention is suitable for connecting withdifferent communication schemes and performing handover between networksadopting different communication schemes.

1. A communication terminal apparatus comprising: a call control sectionconfigured to perform call control including call establishment and calldisconnection for each of plural networks that are in accordance withdifferent communication schemes, respectively; and a handover controlsection configured to request a handover from a source network with afirst communication scheme to a target network with a secondcommunication scheme, prevent the call control section fromdisconnecting a call with the source network after a disconnectionrequest to disconnect the call with the source network is received froma server of the networks, when a call with the target network is notestablished in the call control section, after receiving thedisconnection request to disconnect the call with the source network,for a call establishment waiting time period, wait for a notification ofestablishment of the call with the target network while preventing thecall control section from disconnecting the call with the sourcenetwork, allow the call control section to disconnect the call with thesource network, when the notification of establishment of the call withthe target network is received before the call establishment waitingtime period expires, and quit the requested handover after the callestablishment waiting time period expires, when the notification ofestablishment of the call with the target network is not received duringthe call establishment waiting time period.
 2. The communicationterminal apparatus according to claim 1, wherein the handover controlsection is further configured to repeatedly request the handover fromthe source network to the target network, while preventing the callcontrol section from disconnecting the call with the source networkafter the disconnection request to disconnect the call with the sourcenetwork is received from the server of the networks, when the call withthe target network is not established in the call control section. 3.The communication terminal apparatus according to claim 2, wherein thehandover control section is further configured to repeat the request forthe handover using of the plural networks that are in accordance withdifferent communication schemes.
 4. The communication terminal apparatusaccording to claim 1, wherein, when the call with the target domainnetwork is not established in the call control section the handovercontrol section issues a handover-quitting message while preventing thecall control section from disconnecting the call with the sourcenetwork.
 5. A server configured for communicating with the communicationterminal apparatus of claim 1, the server comprising: a handover requestaccepting section the configured to accept a request for a handover; acall disconnection request section configured to request thecommunication terminal apparatus to disconnect the call with the sourcenetwork when the handover request accepting section accepts the requestfor the handover; and a handover request quitting, configured to quit ahandover procedure of the server when no response is received from thecommunication terminal apparatus in a predetermined determined timeperiod that is longer than the call establishment waiting time period inresponse to the request of to disconnect the call with the sourcenetwork from the call disconnection request section.
 6. A handovermethod comprising the steps of: performing call control including callestablishment and call disconnection for each of plural networks thatare in accordance with different communication schemes, respectively;requesting a handover is from a source network with a firstcommunication scheme to a target network with a second communicationscheme; preventing disconnection of a call with the source network aftera disconnection request to disconnect the call with the source networkis received from a server of the networks, when a call with the targetnetwork is not established; after receiving the disconnection request todisconnect the call with the source network, for a call establishmentwaiting time period, wait for a notification of establishment of thecall with the target network while preventing the call control sectionfrom disconnecting the call with the source network, allow the callcontrol section to disconnect the call with the source network, when thenotification of establishment of the call with the target network isreceived before the call establishment waiting time period expires, andquitting the requested handover after the call establishment waitingtime period expires, when the notification of establishment of the callwith the target network is not received during the call establishmentwaiting time period.
 7. The communication terminal apparatus accordingto claim 2, wherein the handover control section is further configuredto repeat the request for the handover by transmitting a messageincluding an identifier indicating that the handover request is not aninitial request.
 8. A server configured for communicating with thecommunication terminal apparatus of claim 4, the server comprising: ahandover request accepting section configured to accept a request for ahandover; a call disconnection request section configured to transmit tothe communication terminal apparatus the disconnection request todisconnect the call with the source network when the handover requestaccepting section accepts the request for the handover; and a handoverrequest quitting section configured to, upon receiving thehandover-quitting message from the communication terminal apparatus,instruct the source network not to disconnect the communication terminalapparatus.